Important components of modern new drug discovery/creation methods that are directed towards a selected protein target present in a human cell include:
1. identification of “hit” compounds which inhibit or activate the selected target protein. (A hit is defined for these purposes as a compound that scores positively in a given assay and may possess some of the effects and pharmacological properties that the investigator desires. In modern pharmaceutical research, however, hits are virtually never final clinical candidates without substantial further modification);
2. selection of a lead compound upon which to base further studies and refinements of the initial hit compound;
3. optimization of a lead compound (whose chemical structure is either related to or identical to the original hit compound) by making a series of chemical modifications designed primarily to improve the inhibitory or activating properties of the lead compound with respect to the target protein, but which may also improve bioavailability, plasma half-life, or reduce toxicity;
4. profiling the spectrum of biological activity of a given lead compound (including an optimized lead) in order to determine its relative specificity and selectivity for the chosen target protein as compared to other non-target proteins, some of which may be closely related to the target protein itself (such as other members of a protein family);
5. preclinical in-vitro and in-vivo animal studies designed to evaluate dosing ranges, carcinogenicity, absorption, distribution, metabolism, excretion, pharmakokinetics, oral bioavailability (if desired), pharmacodynamics, toxicity, and related parameters;
6. clinical trials in healthy volunteers and in patients afflicted with the disease for which the potential therapeutic treatment is thought to be beneficial.
This invention is directed toward a novel approach which substantially improves steps 1-4 as given above. The method can also be used to create and optimize compounds that are substantially more effective and less toxic than typical experimental drugs that have been identified, optimized or profiled using standard, less sophisticated approaches that are currently in use.
The methodology described herein has been developed as part of an intensive effort to develop advanced new pharmaceutical technologies that convert the “drug discovery” process into one more accurately described as a “drug creation” process by inventing predictable, reliable methodologies that provide the skilled investigator with the necessary tools to create new drugs that target specific proteins of importance in human disease while reducing the time and immense costs associated with the drug discovery/development process.
The progressive development of drug resistance in a patient is the hallmark of chronic treatment with many classes of drugs, especially in the therapeutic areas of cancer and infectious diseases. Molecular mechanisms have been identified which mediate certain types of drug resistance phenomena, whereas in other cases the mechanisms of acquired as well as de novo resistance remain unknown today.
One mechanism of induced (acquired) drug resistance originally thought to be relevant in the area of cancer therapy involves increased expression of a protein known as P-glycoprotein (P-gp). P-gp is located in the cell membrane and functions as a drug efflux pump. The protein is capable of pumping toxic chemical agents, including many classical anti-cancer drugs, out of the cell. Consequently, upregulation of P-glycoprotein usually results in resistance to multiple drugs. Upregulation of P-glycoprotein in tumor cells may represent a defense mechanism which has evolved in mammalian cells to prevent damage from toxic chemical agents. Other related drug resistance proteins have now been identified with similar functions to P-gp, including multidrug-resistance-associated protein family members such as MRP1 and ABCG2. In any event, with the advent of the development of compounds that are specific for a given target protein, and less toxic, the importance of P-glycoprotein and related ATP-binding cassette (ABC) transporter proteins in clinically significant drug resistance has lessened.
Another possible molecular mechanism of acquired drug resistance is that alternative signal pathways are responsible for continued survival and metabolism of cells, even though the original drug is still effective against its target. Furthermore, alterations in intracellular metabolism of the drug can lead to loss of therapeutic efficacy as well. In addition, changes in gene expression as well as gene amplification events can occur, resulting in increased or decreased expression of a given target protein and frequently requiring increasing dosages of the drug to maintain the same effects. (Adcock and Lane, 2003)
Mutation induced drug resistance is a frequently occurring event in the infectious disease area. For example, several drugs have been developed that inhibit either the viral reverse transcriptase or the viral protease encoded in the human immunodeficiency (HIV) viral genome. It is well established in the literature that repeated treatment of HIV-infected AIDS patients using, for example, a reverse transcriptase inhibitor eventually gives rise to mutant forms of the virus that have reduced sensitivity to the drug. Mutations that have arisen in the gene encoding reverse transcriptase render the mutant form of the enzyme less affected by the drug.
The appearance of drug resistance during the course of HIV treatment is not surprising considering the rate at which errors are introduced into the HIV genome. The HIV reverse transcriptase enzyme is known to be particularly error prone, with a forward mutation rate of about 3.4×10−5 mutations per base pair per replication cycle (Mansky et al., J. Virol. 69:5087-94 (1995)). However, analogous mutation rates for endogenous genes encoded in mammalian cells are more than an order of magnitude lower.
New evidence shows that drug resistance can also arise from a mutational event involving the gene encoding the drug target (Gone et al., Science, 2001; PCT/US02/18729). In this case, exposure of the patient to a specific therapeutic substance such as a given cancer drug that targets a specific protein-of-interest (POI, or “target” protein) may be followed by the outgrowth of a group of cells harboring a mutation occurring in the gene encoding the protein that is the target of the therapeutic substance. Whether the outgrowth of this population of cells results from a small percentage of pre-existing cells in the patient which already harbor a mutation which gives rise to a drug-resistant POI, or whether such mutations arise de novo during or following exposure of the animal or human being to a therapeutic agent capable of activating or inhibiting said POI, is presently unknown. In either case, such mutation events may result in a mutated protein (defined below as a theramutein) which is less affected, or perhaps completely unaffected, by said therapeutic substance.
Chronic myelogenous leukemia (CML) is characterized by excess proliferation of myeloid progenitors that retain the capacity for differentiation during the stable or chronic phase of the disease. Multiple lines of evidence have established deregulation of the Abl tyrosine kinase as the causative oncogene in certain forms of CML. The deregulation is commonly associated with a chromosomal translocation known as the Philadelphia chromosome (Ph), which results in expression of a fusion protein comprised of the BCR gene product fused to the Abelson tyrosine kinase, thus forming p210Bcr-Abl which has tyrosine kinase activity. A related fusion protein, termed p190Bcr-Abl, that arises from a different breakpoint in the BCR gene, has been shown to occur in patients with Philadelphia chromosome positive (Ph+) Acute Lymphoblastic Leukemia (ALL) (Melo, 1994; Ravandi et al., 1999). Transformation appears to result from activation of multiple signal pathways including those involving RAS, MYC, and JUN. Imatinib mesylate (“STI-571” or “Gleevec®”) is a 2-phenylamino pyrimidine that targets the ATP binding site of the kinase domain of Abl (Druker et al, NEJM 2001, p. 1038). Subsequently it has also been found by other methods to be an inhibitor of platelet-derived growth factor (PDGF) β receptor, and the Kit tyrosine kinase, the latter of which is involved in the development of gastrointestinal stromal tumors (see below).
Until recently, it had not been observed that during the course of treatment with a specific inhibitor of a given endogenous cellular protein that a mutation in its corresponding endogenous gene could lead to the expression of protein variants whose cellular functioning was resistant to the inhibitor. Work by Charles Sawyers and colleagues (Gone et al., Science 293:876-80 (2001); PCT/US02/18729) demonstrated for the first time that treatment of a patient with a drug capable of inhibiting the p210Bcr-Abl tyrosine kinase (i.e., STI-571) could be followed by the emergence of a clinically significant population of cells within said patient harboring a mutation in the gene encoding the p210Bcr-Abl cancer causing target protein which contains the Abelson tyrosine kinase domain. Various such mutations gave rise to mutant forms of p210Bcr-Abl which were less responsive to Gleevec treatment than was the original cancer causing version. Notably, the mutations that emerged conferred upon the mutant protein a relative resistance to the effects of the protein kinase inhibitor drug, while maintaining a certain degree of the original substrate specificity of the mutant protein kinase. Prior to the work of Gone et al., it was generally believed by those skilled in the art that the types of resistance that would be observed in patients exposed to a compound which inhibited the Abelson protein kinase, such as STI-571, would have resulted from one or more of the other mechanisms of drug resistance listed above, or by some other as yet unknown mechanism, but that in any event said resistance would involve a target (protein or otherwise) which was distinct from the drug's target POI.
Accordingly, the ability to treat clinically relevant resistant mutant forms of proteins that are otherwise the targets of an existing therapy would be extremely useful. Such mutated proteins (theramuteins as defined below) are beginning to be recognized and understood to be important targets in recurring cancers, and will become important in other diseases as well. There exists a need for therapeutic agents that are active against such drug resistant variant forms of cellular proteins that may arise before, during or following normally effective drug therapies. A key purpose of this invention is to provide a generalizable methodology that the skilled investigator may utilize to identify hits from high throughput screening (HTS) systems, create and optimize lead compounds, and profile the spectrum of biological activity of such compounds, all without reliance upon older methods such as cell free radioligand binding assays and the like. An additional key purpose of this invention is to provide compounds that may serve as potential therapeutic agents useful in overcoming mutation-induced drug resistance in endogenously occurring proteins.